1. Field of the Invention
The present invention relates to an optical fiber, which has a highly reliable coating layer, in an optical fiber which transmits an optical signal.
2. Related Background Art
A common conventional optical fiber has structure of a core 1, which passes an optical signal, in a center portion as illustrated in FIG. 4, a soft primary layer 13 which is made to cover a glass optical fiber which includes a quartz glass which has a cladding 2 provided outside the core 1, and a hard secondary layer 14 which is further covered on this primary layer 13.
Then, generally, an ultraviolet curing resin with 3 MPa or lower of Young's modulus is used for the above-mentioned primary layer 13, and an ultraviolet curing resin with 500 MPa or more of Young's modulus is used for the secondary layer 14, respectively.
By the way, as for a polymeric material, physical properties change a lot with bordering on glass transition temperature (hereinafter, only Tg). In particular, in an optical fiber, it is devised that transmission loss of an optical signal which transmits the optical fiber may not be influenced further by an external force by using difference between changes in physical properties because of Tg of an ultraviolet curing resin which is a coating material.
Specifically, in order to give a buffering action to the primary layer 13, a material with low Tg is used as a soft material. Thus, a material with Tg lower than operating environment temperature is selected, and in the usual case, a material with Tg lower than room temperature is used. In other words, since operating environment temperature is higher than Tg, the primary layer 13 has a function as a buffer layer by being in a flexible rubbery state at the operating environment temperature.
On the other hand, since a hard layer called the secondary layer 14 has Tg higher than operating environment temperature, the hard layer is used in a glassy state in the operating environment temperature, and hence, the hard layer acts as a hard protective layer.
When such an optical fiber is immersed in water, delamination may arise between the glass optical fiber and primary layer 13.
Of course, when a coloring layer is applied to an optical fiber illustrated in FIG. 4 or an optical fiber with the coloring layer, that is, a ribbon is constructed by arranging two or more optical fiber planarly to apply the ribbon layer to this, there is a possibility that delamination may arise between the secondary layer 14 and coloring layer, or also between the coloring layer and ribbon layer.
When such delamination arises, and particularly, when delamination arises in an interface between the glass optical fiber and primary layer 13, an uneven force acts to the glass optical fiber owing to this to generate a micro bending, and will increase transmission loss.
Then, so as not to generate the delamination between coating layers, careful examination of a material of each coating layer, and particularly, great efforts for securing sufficient adhesiveness in the interface between the glass optical fiber and primary layer 13 are made.
As an example of one aiming at making the adhesiveness of this interface between the glass optical fiber and primary layer improved, there is an invention disclosed in Japanese Patent Application Laid-Open No. 2001-114535 (patent document 1).
In the patent document 1, for the purpose of making the adhesiveness between a glass optical fiber and a primary layer improved, an optical fiber in which the primary layer is formed of a multilayer coating layer is disclosed. Specifically, an optical fiber is proposed, the optical fiber in which an elastic primary layer is formed in a plurality of layers, and the adhesiveness of the interface between the glass optical fiber and primary layer is made to be improved by heightening concentration of a photo-initiator with proceeding into inside layers.
By the way, for an ultraviolet curing resin for the primary layer 13 used for a sheath of an optical fiber, flexibility is requested strongly in order to enhance resistance on a micro bending first. In particular, with supposing a winter season when operating environment temperature becomes low temperature, it is desired to make Tg of the ultraviolet curing resin for the primary layer 13 lower than operating environment temperature of the optical fiber.
The primary layer 13 becomes good in transmission characteristics in low temperature as Tg becomes low, and as becoming flexible, the resistance on the micro bending becomes good. For that purpose, generally, a materials design is made so as to makes Tg of the primary layer 13 low in consideration of low-temperature resistance, and to make molecular weight between crosslinking points large in consideration of flexibility.
Nevertheless, since the coating becomes flexible but a crosslinked network becomes coarse when Tg of the coating material is made low and the molecular weight between cross-links is made large, there is a problem that a so-called bubble arises easily in the primary layer 13 when the optical fiber is exposed to water and moisture permeates the coating layer. Therefore, actually, although the molecular weight between the crosslinking points is adjusted by making the Young's modulus of the primary layer 13 as a guidepost in order to prevent occurrence of this bubble, materials with Tg of −20° C. or higher are used as the primary layer 13 which satisfies this in many cases.
In particular, when a bubble arises near the interface between the glass optical fiber and primary layer 13 and near the glass optical fiber, micro bending loss arises easily. Therefore, a device in view of a material for making the bubble of the primary layer 13 hard to arise has been investigated energetically.